My hens and the soaring eagle above. How do they know?

Hey all,
I noticed this afternoon for the first time that my two adult hens kept eyeing off the sky above them and I started looking up wondering what it was they were looking at and I soon noticed either a hawk or an eagle soaring high above. How do you think they notice this even though the bird is flying so high up in the sky and so quietly? Is it that chickens naturally keep an eye to the sky for predators more than I have realized? I just found it very intriguing .

Chickens have great eyesight in the day- mine are always watching the bald eagles and hawks flying above. But one of my hens must not have been watching very well... She was the smallest of my bunch, and I can only figure a predator bird got her. There were absolutely no signs of it being carried away by anything else.

there's lots of things you do every day that you're probably not aware of, simple as looking both ways before you cross a street. Same for the chickens. The ones that don't keep a good watch get culled from the gene pool.

I watched a documentary on chickens. They use one eye to look for food, the other to look for predators.
Plus, chickens would not exist if they didn't develop a natural instinct to avoid something that looks like a hawk or an eagle. For evolution, the strain that survived and had offspring were more likely to be the ones that avoided such predators. What's interesting is I've seen mine get skittish because of a flock of birds. But one day they saw a hawk circling and their entire mood changed. They ran under cover, crouched down and got very quiet and still even though it was the first time they'd ever seen one. Another display of this inherited knowledge is mine can tell the difference between a vulcher and a hawk even though they've never been attacked by either. They'll ignore a vulcher circling but a hawk makes them very nervous.
Some fears are genetically passed down. Similar to why we humans are often afraid of cockroaches, rats and vomiting because in order to survive, we had to avoid illness.